Fix new tabs overriding default syntax rules

Fixes #103, #87, and #65

Calling `syntax on` will override existing syntax rules with default
ones. This ends up adding a bunch of syntax rules that users have
disabled.

See `:help syntax` for this snippet:

    The ":syntax enable" command will keep your current color settings. This
    allows using ":highlight" commands to set your preferred colors before or
    after using this command.  If you want Vim to overrule your settings with
    the defaults, use: `:syntax on`

Calling `:syntax enable` doesn't appear to work either. It does the same thing
as `:syntax on`. So the solution implemented here is to conditionally set the
filetype and syntax rules when they aren't already set.